Electron discharge apparatus



Oct. 22, 1957 A. H. w. BECK ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet1 Filed NOV. 29, 1951 Y Inventor AJ-LW B EC K Attorney Oct. 22, 1957 A.H. w. BECK ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet 2 Filed NOV. 29,1951 Inventor HW. BEC K B AAM am Unite States Patent ELncrnoN DISCHARGEAPPARATUS Arnold Hugh William Beck, London, England, assigner toInternational Standard Electric Corporation, NewI York, N. Y., acorporation of Delaware Application November 29, 1951, SerialNo. 258,819

Claims priority, .applicationY Great Britain December 1, 1950 Claims.(Cl.` S15-3.5)

The present invention relates to noise suppression'. in electrondischarge apparatus.V The. invention is concerned with means wherebyshot noise fluctuations in the current emitted by a thermionic cathodemay beprevented from causing appreciable. noise voltages inthe circuitsassociated with the electronbeam.`

In present day receivers for use, at; centimetric wave-` lengths withlow level input signals it is usual. to feed the signals direct to acrystal detector or frequency--y changer stage. This is due to thev factthat existing thermionic arnpliliersA are uoisier than silicon crystals.To be; useful in front of a frequency changer' or detector, theamplifier` must, have sufficientl gain so that the noise powercontributed by the; amplifier output circuit and detector input may beneglectedk in comparison with the amplified signal; for this,V purposeagain o db maybev considered sufficient, the total noise of thereceivingv system may then be considered as that due to theY amplifierinput circuit alone.. It followsV therefore, that the' use of athermionic amplifier in front of the detector stage of a receiverbecomes advantageous provided the gain of the amplifier stage can bemade greater than about l()V db and provided the input noise is notworse. than that of the silicon crystal heretofore used. With presentday*I electron velocity modulation tubes, such as the travelling wavetube, it is a comparatively simple matter to arrange for suiiicientgain, but the input noise of these tubes is normally very high. ThemainV cause of the noise generated bythe discharge device is the randomrate of emission of. and the Maxwellian distribution of ve-i locitiesamong the electrons emitted from a thermionicI cathode. We refer to thenoise from these effects'as shot-noise. The remainder of the noise isdueto thermal noise uctuations in the input circuit. We' are notl corr-Vcerned with these'in this speciiication.

1n triodes and the like having inter-electrode spacirlgs'Y smallcompared to the electron transit angle, shot noise is considerablyreduced by space charge effects. We define the electron transit angle as211- times the ratio of the electron transit time at any frequency ofoperation to a period of oscillation at thatV frequency. For thepurposes of the present specification we dene a space charge limitedelectron beam as one in which only a small fraction of the electronsemitted by the thermionic cathode` is drawn off, the remainder returningto the cathode thusV forming a potential minimum in front of thecathodei It' may be taken that a ratio of total cathode emission currentdensity to initial beam current density of 3:1' is sufficient to producecomplete space charge kcurrent limitation, i. e. any further increasein. saturatedY emission will produce only a negligible change in beamcurrent and we shall assume in the present, specification,A unlessotherwise stated, that we are concerned with a cathode total emissioncurrent density of the order. of 10. amps/cm..2 at temperatures of theorder of. l,l0() K. These figures are representative of good modernoxide cathodes.

ICC

Asstatedv above, it is well known that, at least at low frequencies,shot noise is reduced by the effect of space charge; in fact, if in adiode the cathode emission be increased,- the accelerating anode voltageremaining constant, the anode current remains practically unaltered. Atransient increase in cathode emission causes a transient increase ofanode current which, it can be shown, is due, to a major extent, toelectron velocity modulation in the region between the space chargeinduced potential minimum and the anode. Shot noise, for all practicalpurposes, canbe considered as due entirely to velocity modulationeffects in this region between the potential minimum and anode. We shallalso show that at the potential minimum (to be strictly correct, justbeyond this, so that electrons are moving only in one direction),fluctuations in the conduction current are negligible, there beingpresent merely a velocity fluctuation; a bunching effect converts theelectron velocity uctuations into conduction current variations as thebeam progresses beyond the minimum. Space charge effects can reduce theconduction current at one given plane in a manner familiar from thetheory of electron velocity modulation devices.

As is common with analyses of this type we replace the real electronbeam with variable velocities by a single beam with an A. C. velocityequivalent to the mean square value of the variable velocities.

The space charge induced potential minimum is so close to the cathodethat, in most cases, it would bel practically an impossibility to placea grid at the potential minimum. Our analysis of the eect of transittime and shot noise in space charge limited beams shows, however, that avery considerable improvement in noise can be obtained we place a gridwithin an electron transit-angle of one radian from the potentialminimum.

According to one aspect of the' present invention, therefore, there isprovided an electron velocity modulation apparatus comprisingarrangements for projecting an electron beam from an electron gunthrough a space in which the electrons of the beam may be velocitymodulated by` interaction with the electro-magnetic field therein, thesaid gun having a cathode and a grid at the same H. F. potential closelyspaced thereto defining between them a region in which electron beamcurrent is fully space charge limited, the cathode-grid spacing and thebeam accelerating potential being such that the electron transit angle'between the' space charge induced potential minimum and the grid is notgreater than one radian at the highest frequency of the saidelectro-magnetic field.

Although with any given electron velocity modulation tube, the externalcircuit may be adjusted to provide for a` Wide range of variation inelectron transit times in different regions of the electron beam path,it will be realisedV that the present invention necessitates theprovision ofV a tube having an electron gun with specialcharacteristics. Accordingly, they invention provides an electronvelocity modulation tube adapted for the amplification ofVelectromagnetic waves comprising an electron gun having a grid closelyadjacent to the cathode defining therewith a reg'ion shielded fromexternal electromagnetic Yfields of the frequency of th'e Wave to beamplified, the spacing between cathode and grid being such that when thetube current and potentials are adjusted forV the said amplification andso that the electron current inthe saidV region 'is fully spacerchargellimited, the elec-tron transit angle between the space charge potentialminimum and the said grid is not greater than one radian at the highestfrequency of thersaid electromagnetic waves.

The invention will now be more fully described with reference to theaccompanying drawings, in which:

Fig. 1 illustrates. the variation of potential in a space 'charge'limited planar diode.

' potential minimum and anode, electrons are Fig. 2villustrates theeffect of increase in cathode .emiS-M sion current -on the anode currentof such a diode,

Fig. 3 illustrates diagrammatically an embodiment of the presentinvention arranged to elucidate. our analysis, and

Fig. 4 shows diagrammatically an embodiment in whichy 5 anode asVa, thepotential varying with distance between K'and A in the manner indicatedbythe'curve 1 a potential minimum being formed at the planeMdistant d,from the cathode and d2 from the,anode,;;the'potential at this minimumbeing indicated as -L/m; At the cathode,

electrons are emitted at randominstants ofjtimeand with a Maxwelliandistribution of velocities. In lthe region (a) between K `and Melectrons which aregemittedwith insufficient velocity to pass beyond thepotential minimum are returned to the cathode, so that in thisregionelectrons asrqssafr discussed' inv the. region. (b') .l

.moreelectrons than appropriate to the value of the -potential minimum,for theywill have passed the minimum while the retarding potentialwasless thanit now is. During this time interval, therefore, there Vwillalways be more electrons inthe region (b') than will be present dur-Ying any time interval after the slowestelectron emitted immediatelybefore the'step function has reached the anode.V Theseelectrons aresubject to a greater acceleration than they hadbefore, and hence theanode current, being the product of the number of electrons reachingYthe anode per second and their final velocity, will increase beyond-ther'value which it will 'finally attain. Thus', a

L positive pulse of current will occur at the anode. To some extent thiswill be ott-set by a negative-going pulse at the cathode due to thegreater number and velocity of the reiiected electrons, a similarargument obtaining for these electrons in the region (a') as for thosewe have just However,..in practice, the distance d1 isnisuallyYinuchI.smaller than d2, while the are movingin both directions. In the region(12'), between 20 screening effect of the space vcharge at the potentialmini-v Y moving 1in one direction towards the anode and areaccelerated.v The position of the space charge minimum and the meanvalue of .-Vm depends upon the cathode temperature and the ratio oftotal cathode emission Is to anode ,current Ia. The values of Va and d2for any given', anode current are interdependent, so that if one begiventhe other can have but one Yvalue in order to maintain the saidanode current. Thus, if Va be maintained constant by means of anexternal battery, andthe cathode emission 3 0 bev increased e. g. by asudden change in Workfunction, the. anode current tends toremainconstant. VAn additional number of electrons, however, are emittedby the cathode, which causes the potentialminimum to be de; pressedto alower value. of thepotential minimum is shifted towards the anode,t'

but to'such a small extent that its changein position can be neglectedforjour purposes,-.the potentialdistribution beingaltred as indicatedbythe dotted curveZ in Fig.y 1;

In Fig-2 we show'cur'ves relating current I and time 40 for the cathodeemission current and the anode currentl when, dueto any cause, thecathode emission is suddenly changed from Is to Is-i-Als as shown by thecurve 3. We

c'an consider the effect of this qualitatively'as follows. First thepotential of the minimum is made more negative; it follows that there isan increase in the number ofelectrons which have insufficient initialvelocity to vreach the anode.l Had the potential minimum not Vbeen mademore negative, the increase in emission would have led to aproportionate increase in the number of electrons reaching Y the anode.

. Instead of this, however, under the conditionsV where Is la, ,thedepressionl of the potential minimum is s uch that a greater fraction ofthe .total number of electrons emitted from the cathode is reflectedbythe po tential minimum; hence ,the increasedemission causes a netdecrease in the total number of electrons reaching thel anode. Theseelectronshowever,V since the potential difference between M and A isincreased, areY subject to a greater Vacceleration than before, so thatthe anode cur-v rent remains substantially" unchanged but for atransient.60

pulse. It maybe shown that the proportionate change. of anode currents'Ala, is related tothe proportionate change of emission currents AIS bythe equation (Le) "I 3M" .A If Ie z-) ZeU/'a-i- Vm) Is where Boltzmannsconstant, 1.38 l0 Li3 ljoules/V Tis the! cathode temperature in degreesKelvin, and "e is the electronic charge, 1.5 9 l019 Coulomb. f

"In Fig.. 2 the variation of anode current Ia isl indicated 70 Vby thecurve 4. The anode curreiitpulse V5 shownl in Fig. 2 is to beexpectedfrom general principles, but the mediatej- 'factors involved warrant.further discussion. Dpring the time Ywhile they potential minimum ischanging its value, at any instantin the region ;(b) `there will be r`At the same time the position 3 5 "i transit effect thereforeisa'positive pulse such as 5 shown in'Fig. 2. From the above discussionit will be appreciated 25 thatthe major causeof the transient uctuationof anode current due to the sudden increase in cathode emission is avelocity modulation effect in the region (b')., Asshown above, generalconsiderations lead us to enpectuctuation noise in a diode to beconsiderably fact, well known inV short electronv beams. :In a shortelectronbeaml in lthe absence of space charge, the mean upon :the meancur-'rent-'Imamps emitted by a Vthermionic cathodefwithinthe lfrequencyband VAf`cycles/sec. is given (in th'absenceof space-charge) by l Y l lizzelff-- (2)y I-f the emission bespace charge limited, 'l however, it'is'. known that the'mean square fluctuation current is reduced to 2 .yY

' 1=2e10r2af (3)1 where* is'a smoothing factor, lying between O and l 45depending upon the space charge, but having a value of about 0.1 forImost triodesv at vfrequencies up to about mc./ s. In what follows Weshall introduce a factor 10, a function of transit 4angle 0, applicablefor longer electronl` beams. y Y Y From a-quaiitative consideration ofthe stepfunction. increasein cathode-emission represented in Fig. .2,weV have concluded that'the anode current pulse 5 is due toV avelocityvariation effect rather than a number variation effect. We haveanalysed. the case represented in Fig. 2 Iand'find that we can representthe anode current iluctuation Vby means of an, equation similar toEquation 3 but in which I2 is given by the expression f Non-mier Y- 1eva+vm meanv square conduction current just beyond the spaceV charge.induced potential minimum has Ybeen smoothed by `a factor' of the orderof T4. Wegconclude, therefore, that the effect of the space chargeminimum is to elimi-l nate virtually-all V,conduction current variationsAat the f. minimum;leaving only electron velocity modulation which`produces by drift action the'conduction current variation eventuallyobserved at the Vanode plane.

In order to analysethe transit time elects of shot noise in V electronbeams applicableV to electron velocity modula-w I tion apparatus, wehave used Llewellyns electronic equations, Ywhich are` applicable totheY analysis of conditions between infinite planar electrodes for anydegree of space reduced Vbythe presence of space. charge, and this is,in

value ofv the tiuctuationcomponent superimposed Y charge limitation andin which A. C. variations of potential, conduction current and electronvelocity are assumed to be superimposed upon average or D. C. values ofthese quantities. We take the initial plane of emission as that of theplane M of Fig. l and consider the case where there is zero H. F.potential difference between the two planes, the initial conductioncurrent variation being negligible and the initial electron velocityvariation being given by la known expression previously used in diodenoise analysis.

In Fig. 3 we show very diagrammatically a two resonator klystronarrangement embodying the present invention. An evacuated envelope,indicated by the dotted line 6, to which is sealed a pair of resonators7 and 8 joined by a drift tube 9, encloses an electron gun comprising acathode 10, heating arrangements for the cathode being indicated at 11,and a closely spaced grid 12 parallel to the emitting surface of thecathode. In practice other electron gun electrodes following the gridmay be desirlable but are omitted from the drawing to avoid confusion.The electron beam, after traversing the resonators 7 and 8, islcollected by the collector electrode 13. Resonators 7 and 8 areprovided with the usual input and output wave feed arrangementsindicated by the coupling loop and coaxial line attachments 14 and 15respectively. The grid 12 is shown connected to the cathode 10 by meansof a capacitance 16, which will normally be intrinsic to the mounting ofthe grid, and which is such that grid 12 is maintained at cathodepotential at 'all frequencies within the band-width of resonator 7. Grid12 should be so constructed that it imposes upon the electron beam aunipotential surface orthogonal to the electron trajectories and formswith the emitting surface of cathode 10, so far las is possible, aregion bounded by a pair of planes which in the analysis may beconsidered to be of infinite extent. It is here pointed out thatalthough our analysis applies strictly only to the infinite planarelectrode case, the separation between cathode and grid is so small atmicrowave frequencies that in the present invention these electrodes,provided they be parallel to one another, may be curved withoutdetriment to the performance of the system.

Cathode is connected to the negative pole of potential source 17, thepositive pole of which is connected to the collector electrode 13. Grid12 is shown also connected to the negative pole of source 17 throughdecoupling resistance 18 and biasing potential source 19, while the wallof resonator 7 is shown connected to a tapping point on the potentialsource 17. For the purposes of the following analysis in Fig. 3 we haveindicated `at (a) the plane of the potential minimum in front of thecathode 10. This plane, together with plane (b) coinciding with the grid12, deiines a region (l) in which the current is completely space'chargelimited. A region (2) is defined between the boundary planes (b) and(c), the latter plane being 1ocated at the centre of the inter-actiongap 20 of resonator 7. It is assumed, for present purposes, that theinteraction gap 20 is bounded by grids 21 and 22 which ensure that theiield within resonator 7 in the absence of an input signal at 14 may beinfluenced only by the conduction current of the electron beam. Y

It may be as well to point out at this stage that in previous analysesof shot noise in high requency diodes, the cathode and collectorelectrodes have both been within the boundary of the resonator. In thepresent case a'll the electrodes affecting the beam before it enters theinteraction gap 2@ are maintained at the same high frequency potential,while, at the space charge minimum the total current is equal to theconduction current. It follows, therefore, that we are concerned onlywith conduction current variations and electron velocity uctuations. Wehave found that the latter are proportional to the distance between theplanes (a) and (b) if the transit time involved is short and that, ashas been shown above, at the plane (a) the conduction current variationsare negligible. Hence, if plane (b) be very close to plane (a),

6 the full eitects of the space charge smoothing of the shot noise aremanifested at plane (b). Ideally, therefore, we should like 'to placethe inter-action gap 20 very close to the space charge minimum, in whichcase We should expect to obtain ya shot-noise smoothing factor of thesame order as is 'obtained at low frequencies. In general, this isphysically impossible, but it is possible to place a grid 12 very closeto the space charge minimum. Immediately to the right of the grid 12, i.e. to the right of the 'plane (b), therefore, we obtain the effect ofspace charge smoothing, and in the region (2) between planes (b) and (c)we need not maintain the voltages such as to conform to complete spacecharge limitation of the beam circuit, and, in fact, we ind that, ingeneral, the noise at plane (c) notwithstanding an electrically longregion (2), is very much less than it would be in the absence of thegrid 12 whether or not there were complete space charge limitationbetween the planes (a) and (12); without the grid 12, if the regionbetween planes (a) and (c) were not space charge limited, we could notexpect much space charge smoothing; with grid 12 placed according to theinvention there is imposed, at a plane intermediate the cathode and theutilisation plane, a region of zero A. C. potential which causes theeffect of the large transit angle in 'the region (2) to become small andWe tend to obtain at the plane (c) a noise condition not very much worsethan that at plane (b) adjacent the space charge minimum.

Llewellyns rst order electronic equations upon which our analysis isbased are given below in slightly modified form: Y

1:('Vb- Va) '1n-i" gaizrlveais qz.=(Vb-VQMn-ljqaan-l-vaanvb:(Vb'Va)llail-Qafl32lva 1sa where I is the total alternating current,q is the A. C. component of the conduction current, v is the A. C.component of the electron velocity and V is the alternatingv voltaoe atthe plane (a) `or (b) as indicated by the suix. The suffixes to q and vdenote similarly the planes to which these quantities apply; the as arecoetiicients involving the D. C. conditions, degree of space charge andtransit time; the coeicients will have diiferent values, therefore,according to whether we are considering the region (l) or the region (2)and when it is necessary to distinguish these we shal'l add anappropriate index to the coeicient a. The values of the coefficients interms of transit angle 0 and the velocity u at the appropriate planesare simply related to the coeicients tabulated in Tables I and II of thepaper Vacuum Tube Networks by F. B. Llewellyn and L. C. Peterson inProceedings of the Institute of Radio Engineers, for March 1944, atpages 148 and 149, by transforming Llewellyns Equation 5 into the formof our Equation 5 above. In both the regions (1) and (2) indicated inFig 3 the Vs vanish, and for region (1) we put the conduction currentvariation at the potential-minimum equal to zero, i. e. qa=0 for region(2.), so obtaining:

qu:(1 02H0--axiqbamLl-aaazvbl (7) Substituting for qs and v3 from (6),Qe=va(1-0l2)[(1'a1)a23a222iasa1a232l (8) According to the presentinvention grid 12 is placed close to the planeet the potential minimum,so thatpi, the

transit angle' of region (1), is small. If the D. C. current density bedenoted by Io, evaluation of the coeicients for small gives us:v A

' this result might appear unreasonable until we remember that we stillstipulate thatv the region between the -planes is `to remain spacechargelimited, which condition, for any given value of the direct current andtransit angle between the p'lanes (a) and (b) fixes the relationshipbetween'the other components at the two boundary planes.

. Thus, Vtheeiect is merely one of space charge de-bunchingand'rebunching. p In theY region (2) the direct current is (1 -a1) timesthe'direct current In in the region 1), except to the right of grid 20where it is reduced by the further factor (l-az), and if we assume thatthe degree of space charge is small,

and

. 92 eXP .7'92) where uc=2e/mV2,'Vz being the D. C. potential at plane(c). l Y

' `It is to be notedV that, except for special conditions of spacecharge which need not concern us here, the same values for the abovecoeicients a2 would have .been obtained hadwe assumed the degree ofspace charge arbitrary but made 02 large.

InfEquation 8 we substitute the above values for the coeicients atogether with the substitution for va of the root mean square noisevelocity variation at plane (a) as given by the following knownexpression:

4K7 e 1r 2: un I0 m 1 (See, for example, A. I. Rack. Shot Noise inDiodes, Bell System Technical Journal V17 (1938), page 592.)

` We thus obtain for the mean square noise conduction current at plane(c):

, gere-ae #ein For present purposes we may put u12=2b v. +v

ulg@

Vmay dene a frequency dependent smoothing factor T02 Y which, if a, anda2 0 is given by Y In order to .compare thisv result with the noisefactor obtainable with usual present Vday velocity modulation Y Y l." Ifr tube designsV we must evaluate the coeicients o'czi and usal forregion 1) for the case where 01 is large.

F0129, large we have Y which is identical with the result of otherworkers obtained Vby different methods in the investigation of noise ina long electron beam. Y

As a measure of 'the improvement to be gained by the present invention,let us consider the values of I for an electrode arrangement as inFig.r3 `of the accompanying drawings for lthe two cases Where theseparation between cathode 10 and grid 12 is small, in accordance withthe invention, and where it is large, as obtaining in prior electrongun'arrangements incorporating a control grid. For convenience we shallassume that the grid 20 is at the same potential as grid 12 and alsothat the interception factors are zero. Typical results are Case 1 Case2 f 01:3.0; 62:12.() radians I"62=19, Io2=0.90Y u Y In order to evaluatethe spacings,V and voltages required in a practical embodiment of theinvention, it is important that We should not neglect the'separationbetween Y Vcathode and potential minimum as is done in many transit timeapproximations; this would lead to results giving us smaller clearancesthan are, in fact, required.V We should also take. into account theinitial velocity'of the electrons at the space `charge minimum. For anygiven transit angle having a corresponding transit time t between spacecharge minimum and grid Y12 the distance d2 between space charge minimumand grid is given by aff/ m Y d2- GEOIU-l-uat Y when e, is thepermittivity of free space Y Vl -9 aefrX 10 fared/metre and where la isthe mean velocity of the electrons leaving the plane ofthe potentialminimum.

5 If V be the required equivalent voltage of the grid 12,

It should be mentioned that, in order't obtain agreementV between thevalues of V and d2 given by Equations 17 Vand 18 with the correspondingvalues given by Langmuirs but must be taken as a Yfunction'of VV-lV-Vm,having a mean value for voltages between 0.1 and 1 volt ofv In order toevaluate zu,L more exactly and to obtain Vm andVi AWe use Langmuirs (E,11) tables and let Y qi i uzzIGT where A is given, for low anodevoltages, to a sucient approximation by the relation For a givenVconstruction: we may'A obtain the average transit angle between spacecharge minimum and grid for any given current density In, and, cathodetemperature T by determining Langmuirs E; in terms of` 11, from thetables, and hence :3, and 11,. .using the above expression for A andsubstitutingrin the expression;

1.5X103 5 103 104 amps/metre?. 1.325X105- 6.83)(105.. 4.48X10-l metre.

42 0.30 0.23 volt.

1.0 4.50 volts. 3.52X10-5-. 5.57 l0 metre. 4.20)(10-5-. 6.05X105 metre.

Assuming the same value of cathode emission current and a fixed spacingof 1.6)(10*5 metre, the frequency fo at which this separation gives atransit angle of l radian between space charge minimum and grid variesfor different values of beam current Io as follows:

Io fo 104 amps. lmetre?.

1.5Xl03-. 3X103 42 3,105 megacycles/sec.

An embodiment of the invention as applicable to a helix type oftravelling Wave tube is shown in Fig. 4. In this embodiment the envelopeis divided into two portions, 23 and 24, sealed to either side of acentrally apertured disc electrode 25 which forms the nal anode of theelectron gun. A screen grid type of electron gun is indicateddiagrammaticaliy, the cathode heating arrangements 11 and the grid 12being connected as described With reference to Fig. 3, the capacitance16 between grid 12 and cathode 10 being shown for convenience externallyof the tube. Cathode and grid assembly are housed within the screeningelectrode 26. The helix 27 of the travelling Wave tube is connected by astraight section of rod 28 to the disc electrode V25 at one end and to acollector electrode 29 at the other. The ends of the helix project intowave guides 30 and 31 respectively, as in known travelling wave ampliersand a magnetic focussing solenoid 32 is provided around the helix. Thedisc 25 clips into an annular member 33 within the wave guide 30 so thatdisc 25 and member 33 together constitute a door-knob probe pick up forthe helix. This arrangement is adopted, apart from its other advantages,so as to bring the electron gun as close as possible to the interactionspace and so reduce the transit angle between grid 12 and anode 25. Grid12 is polarised with respect to cathode 10, as in Fig. 3, by connectionthrough the decoupling resistance 18 to the bias source 19, the screengrid 26 is polarised by source 3d which is in series with source 35, andthe collector electrode 29 is shown connected to a tapping point on thesource 35. The screen and nal anode potentials of the electron gun,together with the potential of grid 12, must be chosen so that theregion between the cathode and grid 12 is fully space charge limited,but after passing the grid 12, the beam may be accelerated ordecelerated as required. A

In the present invention shot noise reduction in an electron beam isobtained by-using a grid adjacent to the cathode to maintain the regionbounded byl these electrodes fully space charge` limited, the transitangle between space charg'einduced potential minimum and grid beingsmall. As has been shownbyfthe examples quoted, at microwave frequenciesthis criterion leads to very close inter-electrode spacings. FromEquations 14 and 16, as an alternative to the present invention-it maybe suggested that the transit angles in the regions I and 2 of Fig. 3,or in the equivalent regions in Fig.,4, should be madesuch thatcancellation occurs. This can be done by adjusting the magnitudes of" 01and 02 so that F02 is minimised. It must be remembered however, thatwhen cancellation is obtained by adjustment o f the electrontrajectories in twoY separate regions itis'not sui'licient for theaverage electron trajectory to obey the cancellation condition. Eachindividual electron must travel along a path obeying the condition. Itis found in practice that such systems of cancellation only produceminor improvement in noise factor because the above mentionedconsiderations involve a degree of electron optical homogeneity which isnot obtainable. For instance, in any real non-infinite plane parallelbeam radial space charge forces exist and cause axial electrons totravel more slowly than peripheral electrons, so that it is impossibleto satisfy the required condition over the whole beam.

In the invention described use is merely made of space charge debunchingeiects in a single region which can be made to approximate the idealplane parallel system rather closely. Beyond the space-charge forminggrid it is not material if a variation in transit angle exists.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modiiicationsthereof, it is to be clearly understood that this description is madeonly by Way of example and not as a limitation on the scope of theinvention.

What I claim is:

l. An electron velocity modulation apparatus comprising means deining aspace having an electromagnetic field therein, electron gun means forprojecting an electron beam through said space to produce interaction ofthe beam with the electromagnetic iield and velocity modulation of saidbeam, said gun having cathode and grid positioned outside said space,means for maintaining said cathode and grid at the same high frequencypotential, means for producing fully space charge limited electron beamcurrent throughout a region defined by the space between the cathode andgrid, said grid being mounted at a distance from the potential minimumof the space charge produced in said region in greater than one radianof the electron transit angle at the highest operating fre- Y quency ofsaid field.

2. An electron velocity modulation tube for the ampliiication ofelectromagnetic waves comprising means deiining an interaction space forsaid electro-magnetic waves and an electron beam, an electron gun havinga cathode and a grid closely adjacent said cathode dening therewith aregion shielded from external electromagnetic tields of the frequency ofthe wave to be amplilied, said cathode and grid being positioned:outside said space means for producing throughout said region fullyspace charge limited electron current, said grid being mounted at adistance from the potential minimum of the space charge produced in saidregion in greater than one radian of the electron transit angle at thehighest operating Jfrequency of said lield.

3. Apparatus according to claim 2 including means for guiding theelectromagnetic waves to be amplified along the path of the beam tointeract continuously therewith.

' priemgmeans providing for sudn'giheWavess'helixsurrqunding theapathbfthe beam. a'rwave guideprobe terminating-Said' helix at. .input 4sind,Said-e1etr0ngun indudingal anode frqmwhich Saidrprobeextends.;.-; Y .r`r5- Ah electron 'Yvelcity modllation, apparatus, ,00u11 Y a vspew@Yhvig an alternating elecfrO-magilstic eld ,thereinl anY electron :gun,for Ypro-Y hiding-.al1 eleqrolrbeam'throngh said spacefsor imeraftism,e

with saidleldgiisaid Agun Vhaying cathode 'and aigrid @Quilted QutsdSaid'spacernd means provdnga .beam

accelerating potential -1 whereby VYa Aspiace charge induced potentialminimum is produced between saidcathQle and said Yaccelerating-potentialprpduingvmeans, saidlathode and grid being Aclosely Yspaced with4respect ne another;

to provide a region in which the electron beam current is substantiallyf ully space charge limited, said grid being greater than one radian ofthe electron transit anglet the highest operating frequency of saidfield and means for intercupling'said-cathode and grid'to maintain themvat' substantially the same high frequency potential.,`

, j *.leflfenceslite'd in the le of this patent f :UNITED STATES PATENTSt 2,280,824 Hansen et a1 Apr. 2s, 1942 l2,484,643 Y vPeterson f Oct. 11,1949 2,519,420 Varian ,.c Aug-20, 195Q 2,584,597 Landauer'; Feb.5, 1952OTHER REFERENCES p Travelingwave Tubes, by LR, Pierce, Ypublismiwby D;Van Nostrand C'o.,'Inc.', N. Y., 1950.4 Y'

